Stabilizing pressure and flow conditions in a screening apparatus

ABSTRACT

A method of stabilizing pressure and flow conditions in a screening apparatus for treating a pulp is described, the pulp being supplied continuously to the screening apparatus through an inlet and passed through a pulp space along a screen of a screen body and discharged continuously through an outlet, said screen body being moved axially with a forward-going movement during an extraction phase and a backward-going movement during a backflushing phase. The flowrate of the pulp through, and the pressure conditions in, the treating zone of the screening apparatus are controlled by altering the volume at the inlet end and outlet end of the screening apparatus during the movement of the screen body. A screening apparatus for carrying out the method is also described, which comprises a cylinder housing with a piston movable to and fro therein by means of a drive device, said piston defining a first chamber filled with liquid and a second chamber filled with pulp, said pulp chamber being in direct communication with the pulp inlet. Furthermore, a connection channel filled substantially with liquid is disposed between the liquid chamber and an outlet zone of the pulp space located at the outlet for treated pulp.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method of stabilizing pressure andflow conditions in a screening apparatus for treating a pulp of fibrouscellulosic material. The invention also relates to a screening apparatusfor performing this method.

U.S. Pat. No. 4,396,509 describes a screening apparatus of pressurediffuser type with a screen moving in a cycle to and fro and beingjournalled on two cylindrical support bodies having different diametersat the inlet and outlet sides of the displacement zone. At the returnmovement of each cycle the screen executes a rapid return stroke, thevolume of the filtrate or extraction space being reduced as a result ofsaid differences in the diameters of the support bodies. The screen isthus backflushed with a liquid volume corresponding to the difference incross-sectional areas of the two cylindrical support bodies multipliedby the stroke length of the screen minus the quantity of displacedliquid removed from the screening apparatus during the return stroke.During the rest of the cycle, i.e. the extraction phase, the screenmoves in the same direction, and at substantially the same speed, as thepulp. In this known method the pulp is moved at the correct speed inrelation to the screen at the same time as clogging of the screen holesis prevented. However, the distribution of backflushing is notsatisfactory. One solution to this problem is to design the screen as atruncated cone, as described in U.S. Pat. No. 4,368,628.

In the known screening apparatus described in the above-mentionedpatents, the pulp flows continuously into and out of the displacementzone at the same time as displacement liquid is supplied radially fromthe outside in the direction to the screen surface and extracted ordisplaced liquid is allowed continuously to leave the apparatus. As aresult of the in and out flow of the pulp the pressure in the pulp willincrease at the inlet end of the screen or displacement zone and drop atits outlet end during the return stroke of the screen.

A pressure gradient thus exists between the inlet end and outlet end ofthe displacement zone. This circumstance may prevent the backflushingfrom being correctly distributed since no corresponding pressuregradient occurs in the space for the displaced liquid. Most of thebackflushing will therefore occur close to the outlet end of thedisplacement zone, whereas little or no backflushing will occur in thedirection to the inlet end of the screen.

SUMMARY OF THE INVENTION

The object of the present invention is to provide stabilization of thepressure and flow conditions in the screening apparatus, to ensureuniform distribution of liquid during the return stroke whenbackflushing the screen.

The present invention relates to a method of stabilizing pressure andflow conditions in a screening apparatus for treating a pulp of fibrouscellulosic material, comprising the steps of supplying pulp continuouslyto the screening apparatus through an inlet, passing the pulp through apulp space along a screen of a screen body, discharging the pulpcontinuously through an outlet, extracting liquid and possibly supplyingdisplacement liquid, said screen body being moved axially with a forwardmotion during an extraction phase and a backward motion during abackflushing phase, and controlling the flowrate of the pulp through,and the pressure conditions in, the treating zone of the screeningapparatus by altering the volume at the inlet end and outlet end of thescreening apparatus during the movement of the screen body.

In the preferred embodiment of the method according to the invention thespace for untreated pulp at the inlet end of the screening apparatus isincreased during said backflushing phase so that pulp is substantiallyprevented from flowing along the screen, and substantiallysimultaneously the space for pulp being treated in the screeningapparatus is decreased in a substantially corresponding degree byintroducing an equivalent quantity of liquid and/or pulp into an outletzone for treated pulp so that pulp treated in the apparatus is caused toflow in the direction to and through the outlet. Furthermore, the spacefor untreated pulp at the inlet end of the screening apparatus is thendecreased during said extraction phase so that pulp flows along thescreen, and substantially simultaneously the space for pulp beingtreated in the screening apparatus is increased in a substantiallycorresponding degree by removing an equivalent quantity of pulp from theoutlet zone by some path other than through the outlet of the screeningapparatus.

The invention also relates to a screening apparatus for performing saidmethod for stabilizing pressure and flow conditions therein whenextracting liquid from a pulp of fibrous cellulosic material andpossibly supplying displacement liquid, comprising a vessel and anelongate screen body disposed therein and provided with a screendisposed to be displaced axially to and fro in the vessel by means of adrive device, said vessel having a pulp space with an inlet for pulp tobe treated and an outlet for treated pulp which has passed along thescreen, said screen body defining a space to receive liquid extractedthrough the screen, which is removed from the vessel through an outlet,the screening apparatus further comprising a cylinder housing with apiston movable to and fro therein by means of a drive device, saidpiston defining a first chamber filled with liquid and a second chamberfilled with pulp, said pulp chamber being in direct communication withthe pulp inlet, and a connection channel filled substantially withliquid and being disposed between the liquid chamber and an outlet zoneof the pulp space located at the outlet for treated pulp.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, the invention will be described further withreference to the drawings in which

FIG. 1 shows schematically a cross section through a screening apparatusaccording to the invention during a first part of a cycle duringoperation.

FIG. 2 shows the screening apparatus according to FIG. 1 during a secondpart of the cycle during operation.

FIG. 3 shows a portion of a screening apparatus according to a knownembodiment which lacks the improvements proposed by the presentinvention.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

FIGS. 1 and 2 show schematically a screening apparatus of pressurediffuser type which is constructed in accordance with the presentinvention for the treatment of pulp of medium consistency, i.e. about8-15%, preferably about 10%. The screening apparatus comprises a closed,vertically disposed vessel 1 with a cylindrical external side wall 2 andupper and lower domed external end walls 3, 4. At its upper end wall 3the vessel 1 is provided with a vertically disposed cylinder housing 5,closed at the top, which can be considered as constituting a narrowerextension of the vessel 1 and thus has a smaller diameter than thelatter. The screening apparatus is provided at the top, with an inlet 6at its inlet end for the pulp, i.e. suspension of cellulosic fiber, tobe treated, and at the bottom at its outlet end, with an outlet 7 forthe treated pulp. The outlet 7 is disposed in the lower end wall 4 ofthe vessel. In the embodiment shown, the pulp inlet 6 is disposed in thelower portion of the cylinder housing 5.

A generally bowl-shaped upper support body 8 is disposed inside theupper portion of the vessel 1. The support body 8 is secured, forinstance, by rib-shaped spacers 9 to and spaced from the end wall 3 ofthe vessel. The inverted bowl-shaped support body 8 has a vertical,cylindrical portion 10 with predetermined diameter. A similar, generallybowl-shaped lower support body 11 is disposed inside the lower portionof the vessel 1, said support body 11 being secured by spacers 12 to andspaced from the end wall 4 of the vessel. The right standing bowl-shapedsupport body 11 has a cylindrical portion 13 of predetermined diameter,this diameter being slightly less than that of the cylindrical portion10 of the upper support body 8.

Further, an elongate screen body 14 with circumferential outer surfaceis axially movably disposed in the vessel 1 and comprises a screen 15 inthe form of a truncated cone, tapering in the direction downwards. Inthe embodiment shown the screen body 14 also comprises a cylindrical,tight portion 16 located above and secured to the screen 15. The upperend of said portion 16 is provided on the inside with an annular seal17, sliding and sealing against the cylindrical portion 10 of thestationary upper support body 8. A similar annular seal 18 is disposedon the inside, at the lower end of the screen 15, sliding and sealingagainst the cylindrical portion 13 of the stationary lower support body11. The screen 15 consists of shape permanent sheet-metal with suitablescreen openings.

The screen body 14 is axially movable up and down by means of a drivedevice 19 disposed above the vessel 1. The drive device may be ahydraulic or pneumatic power cylinder with an axial rod 20 extending adistance into the cylinder housing 5. The rod 20 is secured in thehousing by attachment pieces 21 at a radially enlarged or funnel-shapedorifice portion 22 of an axial pipe 23. The pipe 23 extends down throughthe housing 5 and vessel 1, through a central opening in the uppersupport body 8, said opening having an annular seal 24 to slide and sealagainst the pipe 23. The enlarged orifice portion 22 of the pipe 23bears an annular, external seal 25 to slide and seal against the insideof the housing 5. Further, a seal 26 is disposed at the place where therod 20 enters into the housing 5. The pipe 23, axially movable by thedrive device 19, is rigidly joined to the screen body 14 by attachmentpieces 27 secured to the unperforated cylindrical portion 16 of thescreen body 14. During operating the screen 15 moves continuously up anddown apart from the very brief periods of time elapsing at the endpositions of the movement cycle. The screen is thus arranged to be movedin a slow forward motion during an extraction phase, and immediatelythereafter in a rapid backward motion during a backflushing phase.

A space 28 for pulp is defined between the walls 2, 3, 4 of the vesseland the stationary support bodies 8 and 11 axially spaced from eachother, and the screen body 14 extending therebetween, said spaceextending in direct communication from the pulp inlet 6 to the pulpoutlet 7. Internally the screen body 14 and the two support bodies 8, 11define a space 29 to receive the liquid passing through the screenopenings in the screen 15. This liquid is removed from the space 29through an outlet 30 extending through the upper support body 8 and theupper end wall 3 of the vessel 1.

A treating or displacement zone 31, the inlet end of which is designated32 and outlet end of which is designated 33, is formed within the axialextension of the screen 15 in the pulp space 28. Close to the pulpoutlet 7, inside the vessel 1, is an outlet zone 34 which thus forms aportion of the pulp space 28. A scraper 35 is disposed in the outletzone 34, which rotates the pulp to mix it and facilitate feeding outthrough the outlet 7. The scraper 35 is driven by a drive device 36 viaa vertical shaft 37.

The orifice portion 22 of the pipe 23 described above, with its annularseal 25, forms a piston 38 which is movable up and down by the drivedevice 19 in the cylinder housing 5 and which defines an upper firstchamber 39 for liquid and a lower second chamber 40 for pulp. The pulpchamber 40 is so located that it communicates directly with the pulpspace 28 in the vessel 1.

The liquid chamber 39, located above the enlarged orifice portion 22,communicates with the pulp space 28 of the vessel 1 at its outlet zone34 via a central, open connection channel 41 which includes said pipe 23which as such forms a rigid unit with the screen body 14. In theembodiment shown the central connection channel 41 is telescopic.Besides the axially movable pipe 23, it then includes a stationary pipe42 secured by its lower end to the lower support body 11 at a centralopening 43 therein, and with its upper end portion telescopically andsealingly encloses the lower end portion of the movable pipe 23.Alternatively the movable pipe 23 may be elongated a considerable lengthdownwardly and the stationary pipe 42 correspondingly shortened, or themovable pipe 23 may be extended downwardly and passed through theopening 43 in the lower support body 11, sealingly cooperatingtherewith. In this case the lower portion of such an elongated pipe islocated in said outlet zone 34 of the pulp space 28, and the scraper 35must then be moved a short way down or designed differently so that theelongated pipe runs free from the scraper.

A plurality of distribution pipes 44 are disposed outside the vessel,extending around the side wall 2, for the supply of a suitabledisplacement liquid to the pulp via inlets 45. The displacement liquiddisplaces a corresponding quantity of the liquid phase in the pulpthrough the screen 15 to the inner space 29, from which the displacedliquid is removed via the outlet 30. A plurality of shields 49 aresuitably disposed around the inside of the vessel to distribute thedisplacement liquid supplied. The shields 49 extend downwardly from theinlets 45 in order to define circumferential inlet slots. The shieldsare suitably cylindrical as shown in FIGS. 1 and 2.

The liquid chamber 39 and the central connection channel 41 areinitially filled with a suitable fluid material, e.g. water or someother suitable liquid. During operation a small quantity ofsupplementary liquid is supplied continuously to the liquid chamber 39of the cylinder housing through a special inlet 46 at the upper end ofthe cylinder housing 5. Further, a special outlet 48 is disposed at theupper end of the cylinder housing 5 for deaerating the liquid chamber 39and the central connection channel 41 during the initial filling. Asmall quantity of liquid is also continuously removed through thisoutlet 48 during operation in order to prevent any air from collectingin the liquid chamber 39. The amount of liquid thus withdrawn is thenkept less than the amount of supplementary liquid supplied through theinlet 46. Full hydraulic communication thus prevails during operationbetween the liquid in the liquid chamber 39 in the cylinder housing 5and the pulp space 28 of the screening apparatus at the outlet zone 34.

FIG. 3 shows portions of a known screening apparatus of the same basicconstruction as the apparatus according to the invention. The samereference numbers are therefore used to denote equivalent parts andelements. In the known apparatus, the pulp flows continuously in and outof the displacement zone 31 while displacement liquid is simultaneouslysupplied from outside radially in the direction to the screen surface.During the return movement of the screen, i.e. during the backflushingphase of the screen cycle, consequently, the pressure at the inlet end32 of the screen or displacement zone 31 will rise to a value P₁ anddrop at its outlet end 33 to a value P₂. A pressure gradientconsequently exists between the inlet end 32 and outlet end 33 of thedisplacement zone. This situation will disturb correct distribution ofthe backflushing through the screen openings since the dynamic pressureP_(O) in the space 29 for the extracted or displaced liquid is the sameon the two levels. At the return stroke of the screen, therefore, thefollowing pressure relation exists: P₁ >P_(O) >P₂. The equation confirmsthat the backflushing of the screen openings during the return stroke ofthe screen is not uniformly distributed along the screen 15 and thatmost of the backflushing will occur close to the outlet end 33 of thedisplacement zone 31 whereas little or no backflushing will occur in thedirection to the inlet end 32 of the screen 15.

The apparatus according to the invention eliminates said undesiredpressure gradient in the pulp between the inlet and outlet ends 32, 33of the displacement zone 31. The enlarged orifice portion 22 of the pipe23 shown in FIGS. 1 and 2, and its associated seal 25, as mentioned forma piston 38 which is moved down and up by the power cylinder 19. The netarea of the annular piston 38 is dimensioned so that the stroke volumeof the piston 38 approximately corresponds to the volume of pulpentering through the inlet 6 during the return movement, with theaddition of a volume of pulp possibly displaced by the screen body 14during its return movement. An equal volume of liquid will thereby bedisplaced from the liquid chamber 39 in the cylinder housing 5. Thisliquid displacement will be transmitted through the central connectionchannel 41 that is open all the time so that a corresponding displacedvolume of liquid and possibly pulp previously displaced from below, willbe forced out of the connection channel 41 into the outlet zone 34 ofthe pulp space 28. No pulp will thereby flow through the displacementzone 31 during the return stroke of the screen body 14. The inconvenientpressure gradient P₁ -P₂ in the pulp is thus eliminated during thereturn stroke of the screen body.

FIG. 2 shows the screening apparatus according to the invention duringoperation when the screen 15 is subjected to backflushing with displacedor extracted liquid from the space 29 in that the screen body 14 israpidly moved upwardly by means of the power cylinder 19, arrowsindicating the momentary flows of liquid through the screen openings.The volume of pulp simultaneously supplied through the pulp inlet 6,plus a possible volume of pulp displaced by the screen body 14 itself atthe upper end portion, is "stored" in the pulp chamber 40 of thecylinder housing 5, below the piston 38. An equivalent volume of pulp issimultaneously fed through the opening 43 to the outlet zone 34 in thatthe piston 38 displaces the transmitting liquid in the liquid chamber 39and the central connection channel 41 which in turn causes an equal orsubstantially equal volume of treated pulp to flow out through the pulpoutlet 7 and a small portion of pulp fills any "void space" which mayarise under the lower end portion of the screen body 14 when this movesupwardly. The dynamic pressure difference between P₁ and P₂ willtherefore be zero and this can be achieved at varying production ratesby controlling the speed of the screen body 14 during its returnmovement, which in turn directly actuates the momentary flow of liquidthrough the central connection channel 41 and thereby the pressure inthe inlet zone 47 (pulp chamber 40) and outlet zone 34. The embodimentdescribed thus provides an almost optimum distribution of thebackflushing and formation of a uniform film of liquid over the entirescreen surface during this important phase of the cycle.

FIG. 1 shows the apparatus according to the invention during operationwhen liquid is displaced through the screen 15, as indicated by arrows,while the screen body 14 is moved slowly downwardly by the powercylinder 19 at the same speed as the pulp flows through the displacementzone 31. The speed of the screen body 14 is adjusted to the volume ofpulp entering, plus the previously "stored" volume of pulp which is nowdisplaced from the pulp chamber 40 by moving the piston 38 downwardly.At the same time as a volume of pulp corresponding to the volume fed in,is discharged through the pulp outlet 7, a smaller volume of pulp willbe passed up and "stored" in the lower portion of the stationary centralpipe 42. The volume of pulp "stored" in this way at the bottom of thecentral connection channel 41 is equal to the volume of liquid pressedout from the liquid chamber of the cylinder housing 5 during thepreceding backflushing phase of the cycle. The speed of the screen body14 is automatically regulated with the aid of the pressure difference P₁minus P₂ which shall be maintained at a value of zero. By avoidingdynamic pressure differences between the inlet and outlet ends of thedisplacement zone 31 undesired vertical flows in this zone is prevented,and even ensures uniform transverse displacement of liquid all along thescreen 15.

From FIGS. 1 and 2 it can be seen that the liquid in connection channel41 will flow forwards and backwards, the flow direction being oppositeto the direction of movement of the screen 15. As mentioned, only asmall quantity of replacement liquid may have to be supplied through theinlet 46 at the top of the cylinder housing. The liquid in the centralconnection channel 41 functions in principle as a transmitting medium.

In the screening apparatus according to the invention both the pressureconditions and the flow in the pulp space 28 are stabilized throughoutthe cycle. The flow and pressure stabilizing system described improvesthe effect of the return and cleaning strokes of the screen 15, as wellas essentially improving the synchronization of the movement of the pulpand the simultaneous concurrent movement of the screen 15, in that thespeed of the screen 15 is now all the time adjusted to the real flow ofpulp through the displacement zone 31. This is achieved by pulp onlybeing allowed to pass through the displacement zone 31 during that partof the movement cycle as the screen 15 moves in the same direction asthe pulp. A further advantage of the invention is that the flow andpressure conditions in the pulp pipes leading to and from the screeningapparatus are also stabilized.

The screening apparatus according to the invention can also be mountedfor such cases when the pulp flows in a direction other than downwardsas shown in the drawings.

The treating agents normally used in the paper industry for variouspurposes, e.g. water, filtrate, bleaching agent, etc. can be used asdisplacement liquids. The screening apparatus can of course be usedwithout the supply of displacement liquid, i.e. to extract liquid fromthe pulp to thicken it.

The pulp passing through the screening apparatus may be at atmosphericpressure or at pressures over, and even considerably over, 1 atmosphere.

The dashed line in FIG. 1 refers to an alternative embodiment of theinvention in which the connection channel 41a extends outside the vessel1 instead of centrally inside the vessel. The connection channel 41a isformed by a unitary pipe connected at one end to the liquid space 39 ofthe cylinder housing 5 and at the other end to the outlet zone 34 at theoutlet 7 for treated pulp. In this case, the stationary pipe 42 is notutilized and the opening 43 in the lower support body 11 is omitted.Further, the movable pipe 23 is closed at the ends, or replaced by asolid rod.

In the embodiment shown in FIGS. 1 and 2 the truncated cone is disposedso that its smallest diameter is located nearest the pulp outlet 7 ofthe vessel. In an alternative embodiment the largest diameter is locatednearest the pulp outlet of the vessel and in this case the pulp flowsthrough the inner space of the screen and extracted liquid is receivedin the space between the screen and the side wall of the vessel.

Instead of being shaped as a truncated cone, the screen may consist, forinstance, of two flat screen plates, inclined in relation to each other,with either the smallest or the largest distance between the screenplates located nearest the pulp outlet.

Instead of a common drive device 19, the pipe 23 and piston 38 may haveseparate drive devices, the piston thus being separate from the pipe.The drive device for the piston can then be started a predeterminedshort time before starting the drive device for the pipe.

The control of the flow speed of the pulp through the treating zone ofthe screening apparatus, as well as the pressure conditions therein, canalso be accomplished by an arrangement located in or connected to theinlet conduit to the screening apparatus and having the same function asthe cylinder housing 5 with its piston 38 and various chambers 39, 40.The expression "inlet end of the screening apparatus" thus has widermeaning, i.e. the inlet end may be considered to extend from or belocated at said arrangement according to the last-mentioned embodiment.

That which is claimed is:
 1. A method of stabilizing pressure and flowconditions in a screening apparatus during the treatment of fibrouscellulosic material pulp, utilizing a vessel having an inlet for pulp,and an outlet for pulp, a screen mounted in the vessel between the inletand outlet for reciprocating movement along an axis in either a forwarddirection from the inlet toward the outlet, or a backward direction fromthe outlet toward the inlet, a space being formed for pulp movementbetween the interior of the vessel and the screen, and a liquid outletfor liquid extracted from the pulp by passing from the space through thescreen, a pulp chamber defined by a piston adjacent the pulp inlet, anda liquid chamber defined by the piston, said method comprising the stepsof:(a) feeding pulp into the pulp inlet so that it flows in the spacebetween the vessel and screen, and withdrawing treated pulp from thepulp outlet; (b) withdrawing extracted liquid from the liquid outlet;(c) reciprocating the screen along its axis by moving it relativelyslowly in the forward direction and then relatively quickly in thebackward direction so that backflushing of extracted liquid through thescreen takes place during movement of the screen in the backwarddirection; and wherein during backflushing the pulp supplied to the pulpinlet is received in the pulp chamber defined by the piston, the volumeof the pulp chamber increasing in a corresponding degree as the pulp isfed in by displacement of the piston in the backward direction, and asubstantially equivalent volume of treated pulp is fed out substantiallysimultaneously through the pulp outlet by liquid being displaced fromthe liquid chamber, the volume of the liquid chamber decreasing in acorresponding degree as the volume of the pulp chamber increases, andthrough a connection channel filled substantially with liquid, emergingadjacent the pulp outlet in the pulp space, whereby pulp issubstantially prevented from flowing along the screen duringbackflushing, and wherein during extraction pulp is displaced from thepulp chamber by the piston being moved in the forward direction at thesame time as liquid is received in the liquid chamber and pulp is fedinto the connection channel from adjacent the pulp outlet by increasingthe volume of the liquid chamber in a corresponding degree as the volumeof the pulp chamber decreases.
 2. A method as recited in claim 1 whereina small quantity of supplementary liquid is supplied continuously to theliquid chamber and a small quantity of liquid is continuously withdrawnfrom the liquid chamber through an outlet to prevent air from collectingin the liquid chamber and pulp from collecting in the upper portion ofthe connection channel, the quantity of liquid withdrawn beingmaintained less than the supplementary liquid added.
 3. A method ofstabilizing pressure and flow conditions in a screening apparatus duringthe treatment of fibrous cellulosic material pulp, utilizing a vesselhaving an inlet for pulp, and an outlet for pulp, a screen mounted inthe vessel between the inlet and outlet for reciprocating movement alongan axis in either a forward direction from the inlet toward the outlet,or a backward direction from the outlet toward the inlet, a space beingformed for pulp movement between the interior of the vessel and thescreen, and a liquid outlet for liquid extracted from the pulp bypassing from the space through the screen, said method comprising thesteps of:(a) feeding pulp into the pulp inlet so that it flows in thespace between the vessel and screen, and withdrawing treated pulp fromthe pulp outlet; (b) withdrawing extracted liquid from the liquidoutlet; (c) reciprocating the screen along its axis by moving itrelatively slowly in the forward direction and then relatively quicklyin the backward direction so that backflushing of extracted liquidthrough the screen takes place during movement of the screen in thebackward direction; and (d) improving the distribution of backflushliquid adjacent both the pulp inlet and pulp outlet of the vessel bysimultaneously controlling the pressure at the pulp inlet and pulpoutlet of the vessel during movement of the screen in the backwarddirection.
 4. A method as recited in claim 3 wherein the pulp is ofmedium consistency.
 5. A method as recited in claim 3 comprising thefurther step of supplying displacement liquid to the pulp flowing in thepulp space along the screen.
 6. A method as recited in claim 5 whereinthe screening apparatus has a pulp chamber defined by a piston adjacentthe pulp inlet, and a liquid chamber defined by the piston; and whereinduring the backflushing the pulp supplied to the pulp inlet is receivedin the pulp chamber defined by the piston, the volume of the pulpchamber increasing in a corresponding degree as the pulp is fed in bydisplacement of the piston in the backward direction, and asubstantially equivalent volume of treated pulp is fed out substantiallysimultaneously through the pulp outlet by liquid being displaced fromthe liquid chamber, the volume of the liquid chamber decreasing in acorresponding degree as the volume of the pulp chamber increases, andthrough a connection channel filled substantially with liquid, emergingadjacent the pulp outlet in the pulp space, whereby pulp issubstantially prevented from flowing along the screen duringbackflushing, and wherein during extraction pulp is displaced from thepulp chamber by the piston being moved in the forward direction at thesame time as liquid is received in the liquid chamber and pulp is fedinto the connection channel from adjacent the pulp outlet by increasingthe volume of the liquid chamber in a corresponding degree as the volumeof the pulp chamber decreases.
 7. A method as recited in claim 3 whereinthe screening apparatus includes a piston having a first face whichfaces the pulp inlet and is always in contact with untreated pulpadjacent the pulp inlet, and a second face which faces--via liquid whichacts as transmission liquid--the pulp outlet and is always in contactwith transmission liquid; and wherein step (d) is practiced by axiallymoving the piston simultaneously with the screen so that during movementin the backward direction transmission liquid automatically passes,under the influence of the piston, from adjacent but separated from thepulp inlet to adjacent the pulp outlet.
 8. A method as recited in claim7 wherein the screen is a cylindrical perforated body having a taperfrom the pulp inlet toward the pulp outlet so that the space increasesin size from the inlet to the outlet, and wherein step (c) is practicedby moving the screen in the forward direction at approximately the speedof movement of the pulp through the space from the inlet to the outlet.9. A method as recited in claim 7 comprising the further step ofsupplying displacement liquid to the pulp flowing in the pulp spacealong the screen body.
 10. A method as recited in claim 7 comprising thefurther step of continuously deaerating the transmission liquid.
 11. Amethod as recited in claim 7 comprising the further step of continuouslysupplying transmission liquid to a volume in contact with the pistonsecond face, and continuously withdrawing from that volume lesstransmission liquid than is supplied.